Ariens zero turn, B&S 20HP engine surging and vibration


Old 08-09-18, 11:35 AM
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Join Date: Aug 2018
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Ariens zero turn, B&S 20HP engine surging and vibration

Basic Info:
Ariens Zoom 2048 zero turn riding lawn mower
Model #: 915039

Briggs and Stratton 20HP
Model #: 407777-0126-E1

Have had it with this thing. If it weren't for the 20 day lead time for repairs around here I would just take it in but my parents have been sitting on this forever and their lawn needs some cutting bad.

I believe it started as some sort of electrical short. Originally wouldn't crank at all until I replaced the starter solenoid. Once it was cranking itstill wouldn't run, I checked for spark and thought it was the Magneto Armature since I was not getting any spark on right side (if sitting on the mower). Replaced that but the right side was still not getting any spark. Turns out one of the diodes was burnt out. Soldered in two new diodes and great! Spark! It runs!

But blades would not engage. Tested and replaced the PTO switch and perfect! It runs AND it cuts! While I was doing all this, I replaced the spark plugs, air filter, fuel filter, drained and added new oil (my brother decided in all his wisdom to just keep adding oil for whatever reason and the old spark plugs were wet with oil) and ran some Seafoam through the carb (adding the rest of the bottle to the now months old gas).

So she ran great, and my son went through about 3 acres (his grandparents are out of town on vacation at the moment). Not sure what happened next since I was inside checking up on the rest of the house but my son reported an abrupt shift in the riding quality and shut it down with a little backfire. The mower now rides VERY rough. There is a bit of surging and vibration at idle. Mid-throttle and it vibrates A LOT. Full-throttle and it actually vibrates less than at mid-throttle but still bad. This is all without the blades engaged.

My son doesn't recall running over anything. We inspected the area he was in which does not generally have a lot of debris to run over and there was nothing suspicious this time either.

I put the mower up on stands and took off the engine cover. Due to the vibration I thought perhaps either the flywheel or the crankshaft was out of true somewhere. I had pulled the nut for the flywheel to inspect the key previously when it wasn't running but I torqued it down to 140 ft-lbs per specs. Running the engine and looking at both the flywheel and crankshaft, there does not appear to be any wobble. With the engine off (and battery/spark plugs disconnected) I used a pry bar to check for wobble in the pulleys but again, I don't see anything significant.

I have not touched the carbs yet but it is hard for me to associate the heavy vibration with fuel issues. The surging is not as bad, although noticeable at idle. Frankly I'm a little scared to run it at full throttle (or even half throttle) for long but I do not notice any pronounced surging (or it could be the vibration is so bad i just can't pick it out). I poured some more Seafoam into the carbs to maybe clear it out some more. Checking the plugs and they look fine, although not sure if there was enough time for the plugs to show anything. If it's the carbs, I may just order a new carb. The kit is like 2/3 of a complete carb already.

Here is a video:

Help? Thoughts? Advice?

Last edited by zze86; 08-09-18 at 12:52 PM.
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Old 08-09-18, 01:53 PM
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Join Date: Oct 2002
Location: Hamilton County, Ohio
Posts: 4,288
I would be suspecting the fuel system. It's possible that a bit of sediment from the tank or carb is partially blocking a carb jet. I would tear down an clean the carb and drain all old fuel from the tank and fuel line. Add fresh fuel and give it a go.

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